This disclosure relates to a protective device for an LC filter used in a vehicle-mounted electric device.
Known compressors provided for air conditioners mounted on vehicles such as electric automobiles and hybrid vehicles to compress refrigerant include a motor-driven compressor driven by an electric motor. Such a motor-driven compressor includes an inverter (hereinafter, referred to as an A/C inverter) that controls power supply of the electric motor, and an LC filter. The LC filter composed of a combination of a coil and a capacitor functions as a band-pass filter, and is used to remove noise from power supply of the A/C inverter.
In this motor-driven compressor, once the carrier frequency of the A/C inverter falls within the resonance frequency band of the LC filter, electric resonance is generated in a closed circuit passing through the A/C inverter and the capacitor of the LC filter. Generation of such electric resonance increases a ripple current flowing in the capacitor of the LC filter. Hence, if the LC filter is to be used in a condition where such electric resonance is likely to occur, an element of the LC filter such as the capacitor is required to have a considerably high withstand current.
Japanese Laid-Open Patent Publication No. 56-110438 proposes a protective device for an LC filter located between an inverter and a power supply. This protective device changes the LC value of the LC filter in accordance with the carrier frequency of the inverter, thereby avoiding generation of electric resonance due to interference with the carrier of the inverter. As another example, Japanese Examined Utility Model Publication Nos. 62-104578 and 63-020213, and Japanese Laid-Open Patent Publication No. 2001-111373 propose an LC filter capable of changing the resonance frequency band of the LC filter itself by changing an LC value.
If the aforementioned motor-driven compressor receives power from a power supply common to a traction motor for a vehicle, the carrier of an inverter to control drive power for the traction motor (hereinafter, referred to as a vehicle inverter) used in determining the switching pattern of a switching element may be superimposed on power supplied to the motor-driven compressor. If an nth component (n is an integer of one or more) of the frequency of this carrier approaches the resonance frequency band of the LC filter, a ripple current in the LC filter is increased.
Thus, the carrier frequency of a different vehicle-mounted electric device that shares a power supply with the motor-driven compressor should also be considered in determining the resonance frequency band of the LC filter in the motor-driven compressor. Meanwhile, the electrical specifications of a vehicle-mounted electric device differ depending on a vehicle type, so that the resonance frequency band should be determined individually in accordance with the type of a vehicle to which the motor-driven compressor is to be mounted.
This problem is likely to occur not only in a motor-driven compressor but also in any vehicle-mounted electric device that receives power from a power supply common to a different vehicle-mounted electric device having a power converter that converts the voltage or frequency of power from the power supply.